Cracking the AP Biology Exam

11 Animal Structure and Function

VII. THE MUSCULOSKELETAL SYSTEM

Most organisms need some form of support. Many animals wear their support on the outside. They have an exoskeleton—a hard covering or shell. Insects, for example, have an exoskeleton made of chitin. All vertebrates (animals with backbones) possess an endoskeleton—their entire skeleton is on the inside. In addition to ourselves, fish, amphibians, reptiles, birds, and all other mammals are considered vertebrates and therefore have endoskeletons.

THE HUMAN SKELETAL SYSTEM

In humans, the supporting skeleton is made of cartilage and bone. Cartilage is found in the embryonic stages of all vertebrates. It is later replaced by bone, except in your external ear or the tip of your nose. Here’s one thing you should remember: Bone is a connective tissue that contains nerves and blood vessels. Cartilage, on the other hand, lacks nerves and blood vessels.

Bones

Bone is made up of two substances: collagen and calcium salts. Bone is a dynamic tissue that changes shape when osteoblasts (bone-building cells) and osteoclasts (bone-breaking cells) remodel it. Bones are held together by joints, like the ball-and-socket joint in your shoulder. What holds the joints together? They’re held together by tough connective tissues called ligaments. Just remember that ligaments attach bone to bone. Bones not only serve as support but together with muscles also help us move about. The connective tissues that attach muscles to bones are called tendons.

Muscles

There are three kinds of muscle tissue: skeletal, smooth, and cardiac. For the AP Biology Exam, you’ll need to know the differences among the types of muscles.

Skeletal muscles control voluntary movements. You’ll notice that they have stripes called striations. They are also multinucleated. Let’s look at a detailed skeletal muscle:

Organization of Skeletal Muscle

Muscles are made up of muscle bundles, which subdivide into muscle fascicles. Within each muscle fascicle are units called muscle fiber cells. Within each muscle fiber are contractile fibrils called myofibrils. A single myofibril is subdivided, by Z lines, into sarcomeres or contractile units.

The functional unit in a muscle cell is the sarcomere. Inside a sarcomere, there are two protein filaments: actin and myosin. Actins are the thin filaments, and myosins are the thick filaments:

Muscle Contraction

What happens during a muscle contraction? When acetylcholine is released by a motor neuron, it binds with receptors on a muscle fiber and causes an action potential. The impulse stimulates the release of calcium ions from the sarcoplasmic reticulum. Calcium ions bind to troponin molecules, exposing the myosin-binding sites on the actin filaments. ATP (which is bound to the myosin head) is split and P_i and ADP are released. Myosin, now cocked, binds to the exposed site on the actin molecules and actin-myosin cross bridges form. In creating these cross bridges, myosin pulls on the actin molecule, drawing it toward the center of the sarcomere. Then the actin-myosin complex binds ATP and myosin releases actin.

Smooth muscles are found throughout the body: in the walls of blood vessels, the digestive tract, and internal organs. They are long and tapered, and each cell has a single nucleus. They contain actin and myosin but are not as well organized as skeletal muscles. This explains why they appear smooth. Smooth muscles are responsible for involuntary movements. Compared to those of skeletal muscles, the contractions in smooth muscles are slow.

Cardiac muscles are so called because they’re found in the heart. They have characteristics of both smooth and skeletal muscles. Cardiac muscles are striated, just like skeletal muscles, yet they are under involuntary control, like smooth muscles. One unique feature about cardiac muscle cells is that they are held together by special junctions called intercalated discs. Contractions in cardiac muscles are spontaneous and automatic. This simply means that the heart can beat on its own. Here’s one more thing to remember: Both the smooth muscle and the cardiac muscle get their nerve impulses from the autonomic nervous system.

How does a muscle contract? Let’s review the events that occur during muscle contraction. A muscle contraction begins with a neural impulse:

1. A nerve impulse is sent to a skeletal muscle.

2. The neuron sending the impulse releases a neurotransmitter onto the muscle cell.

3. The muscle depolarizes.

4. Depolarization causes the sarcoplasmic reticulum to release calcium ions.

5. These calcium ions cause the actin and myosin filaments to slide past each other.

6. The muscle contracts.

Let’s compare the types of muscle tissues:

MUSCULOSKELETAL SYSTEM QUIZ

Directions: Each of the questions or incomplete statements below is followed by five suggested answers or completions. Select the answer that is best in each case. Answers can be found here.

1. The flow of calcium into cells is essential to which of the following processes?

(A) Activation of pepsin

(B) Thyroid hormone release

(D) Urine concentration

(E) Depolarization

2. All of the following substances are involved in bone remodeling EXCEPT

(A) Vitamin D

(B) Parathyroid hormone

(D) Thyroxine

(E) Osteoclasts

3. Compared to skeletal muscles, smooth muscle cells are

(A) uninucleated, rapidly contracting, and under voluntary control

(B) uninucleated, slowly contracting, and under involuntary control

(D) uninucleated, slowly contracting, and under voluntary control

(E) multinucleated, rapidly contracting, and contain intercalated discs

Directions: Each group of questions consists of five lettered headings followed by a list of numbered phrases or sentences. For each numbered phrase or sentence, select the one heading that is most closely related to it and fill in the corresponding oval on the answer sheet. Each heading may be used once, more than once, or not at all in each group.

Questions 4–7

(A) Cartilage

(B) Tendon

(D) Collagen

(E) Bone

4. Connects bone to bone

5. Connects muscle to bone

6. Embryonic connective tissue found early in life

7. A mineralized connective tissue